Live attenuated simian immunodeficiency virus (SIV) has been shown to be the best candidate vaccine for inhibiting SIV infection. However, the ability of live attenuated SIV to cause AIDS-like disease in newborn macaques raises concern over the safety of a live virus as a vaccine. Thus, in an effort to improve on the live attenuated SIV as a vaccine, we generated a SIVmac239 construct which encodes sequence elements that should facilitate the excision of an integrated SIV proviral genome. In the previous year, we evaluated the ability of this recombinant SIVmac239 construct to replicate in transformed CD4+ T cells and in macaque peripheral blood mononuclear cells (PBMCs). From these in vitro studies we concluded that the recombinant SIVmac239 containing loxP sequence elements and the cre-recombinase gene is capable of replication in both cultured T cells and in primary rhesus PBMCs. This last year, we identified a group of six rhesus macaques for SIV-infection studies. PBMCs were isolated from these animals, infected in vitro with SIVmac239 and analyzed for virus production. From these studies we found that their PBMCs can support SIVmac239 infection and replication, as culture supernatants from these infected cell cultures produced SIV p27 antigen in the supernatant. The identification of animals susceptible to SIV infection is important for these studies, as animals that are resistant to SIV infection would complicate the interpretation of the data. Extensive evaluation of unique live attenuated SIV in rhesus macaques may provide the means for creating a safe and effective vaccine to prevent the progression of AIDS. FUNDING NIH RR00163 (Project 6) PUBLICATIONS None